运行结果不准确怎么造成的

Question

package com.imooc.test;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Test4 {
    public static ExecutorService threadPool= Executors.newFixedThreadPool(10);
   public String date(int num){
       StringBuilder stringBuilder=ThreadSafeStringBuilder.stringBuilderThreadLocal.get();
stringBuilder.append("数字:"+num);
      String s= stringBuilder.toString();
return s;
   }

    public static void main(String[] args) {
        for(int i=0;i<20;i++){
            int s=i;
            threadPool.submit(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                   String num = new Test4().date(s);

                    System.out.println(num);
                }
            });
        }
        threadPool.shutdown();
    }
}
class ThreadSafeStringBuilder{
    public static ThreadLocal<StringBuilder> stringBuilderThreadLocal=
            new ThreadLocal<StringBuilder>(){
                @Override
                protected StringBuilder initialValue() {
                    return new StringBuilder();
                }
            };
}

运行结果为什么9之后的显示的是双结果呢？怎么造成的哎

Answer 1

线程池设置10个，超过10个后会复用之前的线程，你创建StringBuilder的方式应该是根据当前线程创建的

所以当线程复用时，StringBuilder用的是上一次创建的，里面已有上次创建的内容了